Aromatic aldehydes have aldehyde groups having high reactivity, and so have been widely used. Especially, terephthalaldehyde having two aldehyde groups at para position has been gained attention as raw material for medicinal products, agrochemicals, pigments, liquid crystal polymers, or plastic having heat resistance.
As conventional methods for the preparation of terephthalaldehyde, there are dehydration method of intermediate by chlorination of p-xylene, and hydrogenation method of dimethyl terephthalate. However, the above conventional methods are not suitable for economical mass production of terephthalaldehyde since the processes are complicated and are carried out under high pressure and environmental unfriendly condition.
In order to solve these problems, continuous efforts have been exerted into developing a mass production method of terephthalaldehyde by gas phase oxidation of p-xylene with molecular oxygen.
For example, Japanese Patent Publication No. 47-002086 discloses a complex oxide catalyst having the W and Mo ratio ranges of from 1:1 to 20:1. Japanese Patent Publication No. 48-047830 discloses a catalyst comprising V and Rb or Cs. U.S. Pat. No. 3,845,137 discloses a catalyst consisting of two components of W and Mo and one or more component selected from the group consisting of Ca, Ba, Ti, Zr, Hf, Tl, Nb, Zn, and Sn. Also, U.S. Pat. No. 4,017,547 discloses a catalyst consisting of Mo oxide, W oxide or silicotungstic acid and Bi oxide.
However, in case of using the catalysts of these inventions, the selectivity and yield of terephthalaldehyde were low, and so their industrial practicality has been limited.
Also, U.S. Pat. No. 5,324,702 discloses a catalyst distributing a first component selected from the group consisting of Fe, Zn, Zr, Nb, In, Sn, Sb, Ce and Bi, and a second component selected from the group consisting of V, Mo and W, on a deboronized borosilicate crystal molecular sieve by chemical vapor deposition (CVD). This catalyst showed relatively higher conversion rate of p-xylene and yield of terephthalaldehyde than conventional catalysts. However, in case of using the catalyst, there has been a limit to increase the selectivity due to various by-products, and so it has been difficult to separate and purify the by-products accordingly.
Also, U.S. Pat. No. 6,458,737 discloses a catalyst comprising a major component of W and one or more components selected from the group consisting of Sb, Fe, Co, Ni, Mn, Re, Cr, V, Nb, Ti, Zr, Zn, Cd, Y, La, Ce, B, Al, Tl, Sn, Mg, Ca, Sr, Ba, Li, Na, K, Rb and Cs. The catalyst can provide high yield of terephthalaldehyde almost enough to have industrial practicality. However, the catalyst also has limitation in separation and purification since the selectivity of terephthalaldehyde is not so high, compared with high conversion rate of p-xylene. Also, the catalyst has problems in heat stability and life span since it comprises Sb component which is sublimated and lost at high temperature.
In short, in case of using the conventional catalysts, the yield of terephthalaldehyde was low. Or, the separation and purification of terephthalaldehyde were difficult due to low selectivity even though they have high yield. Also, it was difficult to prepare catalysts having homogeneous composition and performance since they had to use complex oxide having multi-component. Further, the industrial practical use of the catalysts has been limited due to their short life span resulting from comprising components whose heat stability is low.
On the other hand, Korean Patent Application No. 10-2004-0089376 filed by the present inventors disclosed a single component catalyst comprising tungsten oxide and fire-resistant inorganic carrier as optional component. In case of using the catalyst, it is easy to prepare homogenous catalyst, and advantageous in that terephthalaldehyde can be prepared in high selectivity and yield, compared with conventional complex oxide having multi-component. However, a catalyst having higher selectivity for preparing terephthalaldehyde has been still required because the future research trend of partial oxidation reaction process field is focused on development of a catalyst which can reduce the green house gas which is major by-product, and can increase the selectivity of an object product, which will be a very important standard for commercialization of the catalyst process.